Concrete is a prehistoric building material which is one of the oldest used by mankind for construction purposes. Recent research indicates that concrete originated in the New Stone Age, around 7000 B.C. (Malinowski, et al., "Prehistory of Concrete," Concrete International, V. 13, No. 3, Mar. 1991, pp. 62-68). Since ancient times, mankind has searched for ways to improve the properties of concrete.
As early as 1600 B.C., man realized the benefits to using natural pozzolans in conjunction with building materials. The Greeks discovered pozzolan-lime mixtures sometime between 700 and 600 B.C., and later passed their use in concrete along to the Romans about 150 B.C. Early pozzolans consisted primarily of volcanic ash.
Generally, a pozzolan is defined as "a siliceous or siliceous and aluminous material which in itself has little or no cementitious value but will in finely divided form and in the presence of moisture chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties" (American Concrete Institute definition 116R). Slag and fly ash, a product of the combustion of powdered coal, are commonly used pozzolans.
A natural pozzolan is either a raw or calcined natural material that has pozzolanic properties. Id. Some natural pozzolans include volcanic ashes, pumicites, opaline cherts and shales, tuffs, and some diatomaceous earths.
The properties of natural pozzolans vary considerably, depending on their origin. This is caused by the variable proportions of the active materials and their mineralogical and physical characteristics. Most natural pozzolans contain substantial amounts of constituents other than silica, such as alumina and iron oxide, and alkalis, which will also react with calcium hydroxide and alkalis (sodium and potassium) to form more complex compounds.
The molecular structure as well as the amount of silica present in pozzolans is also very important. Generally, amorphous silica reacts with calcium hydroxide and alkalis more rapidly than does silica in the crystalline form (quartz, for example) (ACI Committee 232, 1994, "Proposed Report: Use of Natural Pozzolans in Concrete", ACI Materials Journal, V. 91, No. 4, p. 412).
When a mixture of portland cement and a pozzolan reacts, the pozzolanic reaction progresses like an acid-base reaction of lime and alkalis with oxides (SiO.sub.2 +Al.sub.2 O.sub.3 +Se.sub.2 O.sub.3) of the pozzolan. Two things happen: first, there is a gradual decrease in the amount of free calcium hydroxide with time, and second, there is an increasing formation of CSH and calcium aluminosilicates that are similar to the products of hydration of portland cement. The partial replacement of portland cement by pozzolan has been found to increase the resistance of concrete to sulfate and seawater attack which is in part attributable to the removal of free calcium hydroxide formed in the hydration of portland cement by combination with the pozzolan. The end result will be that the concrete mixture will contain less calcium hydroxide and more CSH and other products of low porosity.
The shape, fineness, particle size distribution, density, and composition of natural pozzolan particles influence the properties of freshly mixed and hardened concrete, and the strength development of hardened concrete. Most natural pozzolans tend to increase the water requirement of the concrete product as a result of their microporous character and high surface area. When properly employed as an ingredient of portland-pozzolan cement or as admixtures to portland-cement concrete, pozzolans can improve the performance of both fresh and hardened concrete.
There are several advantages to combining pozzolans with concrete. Firstly, concrete containing a pozzolan typically has lower permeability. Secondly, pozzolans have been used in mass concrete of low cement content to reduce the temperature rise of concrete as compared to a comparable concrete mixture containing portland cement as the only cementing material. The slower rate of heat development with pozzolans permits more economic removal of heat than with comparable non-pozzolan concrete.
Thirdly, it has been reported that almost any pozzolan, when used in sufficient quantity, is capable of preventing excessive expansion resulting from alkali-silica reaction. The alkali-silica reaction involves the interaction of hydroxyl ions associated with alkalis in portland cement with certain siliceous constituents of the aggregates in concrete. Products of the reaction can cause excessive expansion, cracking, and general deterioration of the concrete. Investigators have observed that natural pozzolans are usually more effective than fly ash in controlling the alkali-silica reaction (Pepper, L, and Mather, B., 1959, "Effectiveness of Mineral Admixtures in Preventing Excessive Expansion of Concrete Due to Alkali-Aggregate Reaction," ASTM 59, pp. 1178-1203).
When pozzolanic materials are used to replace cement on an equal volume basis, early strengths may be reduced. However, these early strengths can be increased by substituting the pozzolanic material for the cement on an equal mass basis or a volumetric amount greater than one to one for the cement replaced, provided water content is not increased excessively. The contribution of the pozzolanic strength development occurs sometime after seven days of hydration (Mehta, "Natural Pozzolans", Supplementary Cementing Materials for Concrete, 1987).
Use of natural pozzolans with portland cement in concrete generally increases its resistance to aggressive attack by seawater, sulfate-bearing soil solutions, and natural acid waters. The relative improvement is greater for concrete with low cement content. In one study, it was found that blended cements manufactured using highly siliceous natural or artificial pozzolan, slags, or silica fume perform better in sulfate environments than ordinary portland cement (Patzias, T., 1987, "Evaluation of Sulfate Resistance of Hydraulic Cement Mortars by the ASTM C1012 Test Method," Concrete Durability: Katherine and Bryant Mather International Conference, SP 100, American Concrete Institute, Detroit, pp. 2103-2120).
As a practical matter, the use of natural pozzolans can provide a major economic benefit in that the use of these materials permits a reduction in the amount of portland cement in the mixture. Other technical benefits in the use of pozzolans have not been spectacular, however. While the use of pozzolans often provides higher strengths for concrete at later ages, their early strength is deficient in contrast to pure portland cement. Further, natural pozzolans often contain many impurities which decrease their resistance to sulfates and discolor the concrete.
It is therefore a primary objective of the present invention to provide an improved pozzolan which when combined with concrete products increases early strength of the concrete-pozzolan product.
Another primary object of the present invention is to provide an improved pozzolan with good sulfate resistance when combined with concrete products.
It is yet another objective of the present invention to provide an improved pozzolan which is safe and economical to use.